In cases of chronic kidney failure, various methods of extra-corporeal blood treatment are used to remove substances needing to be excreted and to withdraw fluid. Whereas in haemodialysis (HD), the movement of substances of low molecular weight through the semi-permeable membrane (in the form of diffusion) is determined essentially by the differences in concentration between the dialysis fluid and the blood, in haemofiltration (HF), substances dissolved in the plasma of the blood, and particularly substances of higher molecular weight, are removed effectively by a high rate of fluid flow (convection) through the membrane of the dialyser. In haemofiltration, the dialyser acts as a filter. Haemodiafiltration (HDF) is a combination of these two methods.
In haemodiafiltration, some of the ultrafiltrate withdrawn through the membrane of the dialyser or filter is replaced by a sterile substitute fluid (a substituate) which is fed into the extra-corporeal blood circuit either upstream or downstream of the dialyser. The infeed of substituate upstream of the dialyser is referred to as pre-dilution, and the infeed downstream of the dialyser is referred to as post-dilution.
There are known pieces of apparatus for haemodiafiltration in which the dialysis fluid is produced on-line from fresh water and a concentrate and the substituate is obtained on-line from the dialysis fluid. In the known pieces of haemodiafiltration apparatus, the substituate in the extra-corporeal blood circuit is fed in from the dialysis fluid system of the apparatus via a substituate line. Pieces of haemodiafiltration apparatus are described, for example, in European Application Nos. EP 0 974 371 A3 and EP 1 595 560 A1.
In individual cases, complications may occur in dialysis due to shifts in the levels of electrolytes, leading to headaches or psychomotor disorders in patients. These complications are known as disequilibrium syndrome.
Disequilibrium syndrome is described in German Application No. DE 32 23 051 A1. German Application No. DE 32 23 051 A1 describes a dialysis apparatus in which the composition of the dialysis fluid is regulated during the dialysis treatment as a function of the electrolyte content of the dialysis fluid upstream and downstream of the dialyser. The electrolyte content of the dialysis fluid is measured by sensors arranged upstream and downstream of the dialyser.